This invention relates to a method and apparatus for coating a moving web with a plurality of coatings and more particularly to a method and apparatus for coating a moving web with a multilayer liquid composition which is subsequently set or gelled on the web.
Presently, there is available coating apparatus for applying a plurality of superimposed layers to a moving web. Typical apparatus utilizes a plurality of inclined slide surfaces separated by exit slots through each of which is metered a coating solution onto an adjacent inclined surface. The coating solutions flow by gravity over the inclined surfaces and those metered through upstream slots flow over coating solutions metered through downstream slots and form a multilayered stream formed of the individual coating solutions that cascade over the downstream inclined surfaces. As the coating solutions flow under gravity over the inclined surfaces, each layer becomes smooth and is of uniform thickness. At the last inclined surface or slide, the multilayered stream is stratified in a configuration which constitutes the desired multilayered coating to be contacted with the moving web. The end of the last slide is spaced apart from the moving web so that the multilayered stream exiting the last slide toward the web forms a bead or bridge between the last slide and the moving web. A pressure differential generally is effected across the bead by applying a vacuum to the bottom surface of the bead immediately adjacent the top surface of the web to stabilize the bead against excessive vibration and rupture. As the web contacts the bead, it entrains the multilayered coating, thereby becoming coated.
Coating apparatus of the kind described is useful, for example, to form webs coated with superimposed layers of aqueous photographic compositions including light sensitive materials, chemical sensitizers, antifoggants, developing agents and the like. These compositions are mixed with synthetic or naturally occurring colloids such as gelatin, polyvinyl compounds, or the like, which form nonflowing set layers containing the photographic compositions when the colloid is dried on the web.
Typical of the apparatus described above are set forth in U.S. Pat. Nos. 2,761,419 and 3,220,877. In the apparatus disclosed in U.S. Pat. Nos. 2,761,419 and 3,220,877, the slides are planar and the angle between the planar surface of the last slide and the tangent to the web at the point of coating contact is acute. When coating a web with such apparatus, the feed slots between the slides sometimes become partially blocked with solid impurities. When this occurs, the liquid composition emanating from the slot is disrupted and forms a noncontinuous layer on the slide which is not self-correcting as the layer flows down the slide. When this occurs, the liquid layer immediately above the disrupted layer fills the discontinuity formed by the blockage and the resultant coating on the web is undesirably streaked and photographic film formed from the thus-coated web is unacceptable.
It would be desirable to provide a coating apparatus which assures forming continuous layers within a multilayer liquid composition even when the feeding slots for the liquid become partially blocked.